In recent years, lithium ion secondary batteries have been used as a power source for driving a vehicle, such as a hybrid electric vehicle or an electric vehicle, or a portable electronic device, such as a notebook computer or a video camcorder.
For example, Patent Literature 1 discloses a lithium ion secondary battery using LiPF6 as a nonaqueous electrolyte, and having a concentration of lithium salt of 0.4 to 0.8 mol/L. Repeated charging and discharging of such a lithium ion secondary battery is known to lead to degradation phenomena, including a gradual increase in internal resistance of the battery.